This invention relates to an image scanning device. More particularly, it is concerned with a scanning optical system in such image scanning device having a half tone image forming optical system which records such half tone images, or records and displays such half tone images on an appropriate display medium.
A scanning optical system utilizing a rotatory polygonal mirror and a vibrating mirror has generally been used in various types of recording and displaying devices such as facsimile devices utilizing laser beam, various types of display devices, printing devices, and so forth, due to its capability of taking a large scanning angle, and due to its minimal color dispersion. In particular, when the rotatory polygonal mirror is used in the scanning optical system, it can be widely used as a high speed scanning device.
As a method for recording or displaying a half tone image in such a scanning optical system, there has been known one that utilizes the photosensitivity characteristic of the recording or displaying surface by imparting an intensity modulation to a scanning beam by a conventional light modulator. However, this method of forming the half tone image in utilization of the photosensitive characteristic of the photosensitive part makes it necessary that a method for modulating the intensity of the scanning beam be continuously varied in accordance with the characteristic of the photosensitive part. The characteristic of the photosensitive part is liable to be affected by changes in the surrounding circumstances such as, for example, atmospheric temperature, humidity, and so on, and, particuarly, when the electrophotographic method is employed on this photosensitive part, a remarkable influence due to the static electricity is inevitably. In addition, this method is incapable of avoiding the influence of the drift phenomenon, in which an output from a light source such as laser, etc. fluctuates with lapse of time with the consequence that a stable half tone image is very difficult to maintain over a long period of time.